Recently, lithium ion secondary batteries having high energy density have been widely used as power sources for small portable electronic devices such as mobile phones and notebook-type personal computers.
Such a lithium ion secondary battery is produced through the steps of stacking sheet-like positive and negative electrodes and a separator made of, for example, a porous polyolefin resin film in layers, or rolling them up, placing the resulting layered or rolled structure in a battery case such as a metal can, pouring an electrolyte solution into the battery case, and then air-tightly sealing the battery case.
Various materials and structures have been proposed for porous polyolefin resin films used as battery separators. In particular, a porous film containing polyethylene is suitably used because such a porous film has a property that when heated, the resin melts and fills the pores, thereby allowing the battery to have a so-called shutdown function.
Meanwhile, many studies and inventions have been made on materials for positive and negative electrodes to provide high capacity and high power lithium ion secondary batteries.
In spite of these intensive studies and inventions, however, conventional lithium ion secondary batteries still have a problem such that repeated charge and discharge in a room temperature or high temperature atmosphere may cause a decrease in the capacity, a deterioration of the output characteristics, and a reduction in the safety.
One of the known causes of this problem is that if a porous film separator containing polyethylene is in contact with a positive electrode, the oxidation reaction of the separator proceeds when the battery is exposed to high temperatures in a charging state, resulting in a deterioration of the battery characteristics (see, for example Non-Patent Literature 1).
In order to solve this problem, there has been proposed, for example, a lithium ion secondary battery in which a separator including a polyolefin layer and an oxidation resistant layer is disposed between a positive electrode and a negative electrode so that the oxidation resistant layer faces the positive electrode (see Patent Literature 1). This oxidation resistant layer contains an oxidation resistant polymer. There has been proposed, as the oxidation resistant polymer, a polymer not containing a —CH2— group in a main chain and not containing a —CH(CH3)— group.